It is proposed to determine the mechanism of the proteolytic enzyme, plasmin, which has been directly implicated in the transformation of cells. In particular, the catalytic groups which participate in the active site will be determined, the intermediates in the reaction, both tetrahedral and acyl-enzyme, will be determined, as will the concentration of active sites, the specificity of plasmin, the inhibitors of plasmin, both covalent and non-covalent, and the possible monomer-dimer equilibrium. The last two factors could be important in the control of cancer.